I can see that I can delete unused log files (db_archive [no options])
from the backup. But when is it safe to remove log files from the
active environment?

Let auto-archive do that for you.

Ok.. then I guess, I'm confused by the repeated warnings like:

"To have them removed automatically, place set_flags DB_LOG_AUTOREMOVE
directive in DB_CONFIG. Note that if the log files are removed
automatically, recovery after a catastrophic failure is likely to be
impossible."

How to remove log files without making catastropic recovery impossible ?

You can't. That's why you slapcat periodically so you have an alternative.
Not that I personally have ever hit a catastrophic event in the years I've
been using OpenLDAP where the log files would have helped in any way.
I.e., disk failure? -- logs are useless if they were on the disk that
failed. And in that case, I just slapcat a different replica and slapadd
it onto the server with the failed disk once it has been replaced. I.e., I
find having replicas a much more effective disaster recovery mechanism than
log files that are susceptible to other problems.